Evolutionary history of reef-dwelling and associated lagoonal mollusc assemblages of the uplifted Niau Atoll (Tuamotu, French Polynesia) since the Pliocene

The evolutionary history of the lagoonal molluscan thanatocenoses on the raised Niau Atoll (French Polynesia) is studied in relation to the geological evolution of the atoll and the episodic exchanges between the oceanic waters and those of the lagoon since the Late Pliocene. Three thanatocoenoses are described and compared with the extant fauna of the raised lagoon. The first one, located beneath a limestone pedogenetic crust, was recently dated to the Late Pliocene-Early Pleistocene and reveals a rich assemblage of 92 molluscan species. Seven new species are described, five of which are recognised after examining the shells under UV light: four gastropods Cerithium niauense Merle, n. sp. (UV), Conus (Conus) letourneuxi Merle, n. sp. (UV), Conus (Tesselliconus) georgesrichardi Merle, n. sp. (UV), Bulla niauensis Merle, n. sp. (UV), and three bivalves ?Lunulicardia distorta Merle, n. sp., Indotellina fakaauensis Merle, n. sp. and ?Gafrarium troendlei Merle, n. sp. (UV). The other two thanatocenoses are herein dated to the Mid and Late Holocene. With around ten species, their assemblages are much poorer than those of the Plio-Pleistocene and support numerous local extinctions. These extinctions are regarded as mainly resulting from the uplift of the atoll, which has led to the isolation and drying out of the lagoon to the present day, as well as reduced water exchanges with the open sea. Currently, only three species of molluscs are abundant in the lagoon, two of which were already present in the Plio-Pleistocene, the bivalve Ctena bella Conrad, 1837 and the gastropod Cerithium punctatum Bruguière, 1792, and another gastropod species Clypeomorus brevis (Quoy & Gaimard, 1834) was introduced in the atoll between 2008 and 2020.